With the advent of an aging society, the number of patients with dementia is increasing year by year. There are various types of dementia, and it is important to determine the type of dementia by diagnosis so that the disease is treated appropriately according to the determined type.
To respond to such a demand, an ASL imaging method (hereinafter may be simply referred to as an ASL method) based on ASL (Arterial Spin Labeling) is recently being used as a novel perfusion weighted imaging technique using MRI.
PWI (Perfusion Weighted Imaging) (also referred to as perfusion imaging) is a technique for imaging perfusion, which is the flow of blood through capillary vessels in tissues (see, for example, Patent Literature 1). The state of perfusion can be known by marking blood on the upstream (arterial) side of the tissues and observing the passage of the blood flowing through the tissues.
In a general conventional method, a contrast medium containing a radioactive isotope serving as a tracer is injected into an artery, and the injected radioactive isotope is observed. However, this method has an invasion problem.
The ASL method is characterized in that perfusion weighted images can be noninvasively obtained without any contrast medium.
The principle of the ASL method is as follows. A carotid artery is irradiated with inversion pulses to spin-label protons in blood flowing through the carotid artery, and images of the head are taken after a predetermined time, i.e., after the brain is perfused with the blood. These images are used as labeled images. Similarly, images are taken without irradiation with inversion pulses and are used as non-labeled images.
The above procedure is repeated to obtain a group of labeled images and a group of non-labeled images. PWIs quantitatively showing the distribution of cerebral blood flow can be obtained by the difference between the group of labeled images and the group of non-labeled images.
In addition, absolute CBFs (Cerebral Blood Flows) can be obtained from the PWIs. More specifically, the absolute CBFs are determined by substitution of the time of passage of the labeled blood, a blood-brain partition coefficient, the longitudinal relaxation time of the blood, etc. (see Non Patent Literature 1).
Generally, labeled images and non-labeled images are taken using a fast imaging sequence such as EPI (Echo Planar Imaging). Such a fast imaging sequence allows fast imaging, but the resolution of the images is low. However, when there is a need to take additional high resolution MRI images using a conventional imaging method, an imaging sequence that can provide additional high resolution images may be performed subsequent to the imaging procedure using the ASL method with the subject being secured.
The above-described and other features of the ASL method are summarized as follows:—Perfusion weighted images can be noninvasively obtained without any contrast medium. Morphological images (low-resolution images), together with functional images including PWIs and CBFs, are obtained by the ASL method, and the positions in the obtained images completely match each other. Morphological images (high-resolution images) can be taken using a conventional method subsequent to the imaging by the ASL method, and the positions in the morphological images substantially match the positions in the PWIs and CBFs.
As described above, PWIs and CBFs are very useful in the diagnosis of various brain diseases because the level of local blood flow can be obtained.